The present invention relates to a compressor system and a method for operating a compressor system.
In order to be able to satisfy the compressed air requirements of subsystems of modern commercial vehicles, the compressors belonging to a compressed air supply unit are often provided with a transmission ratio i<1. Here, the transmission ratio i of the speed of the driving shaft nAntrieb, which is identical with the engine speed of the commercial vehicle, to the speed of the driven shaft nAbtrieb, which is identical with the speed of the compressor, is defined as i=nAntrieb/nAbtrieb and is chosen to be as small as possible.
This is advantageous since there is often an increased air requirement from the commercial vehicle, especially at low engine speeds. This is the case, for example, during container interchange or in the case of a bus approaching a stop. The latter must first of all stop there, open the doors and vent the air suspension in order to allow passengers to alight comfortably with the floor of the bus at a low level. The doors must then be closed again and the air suspension resupplied with air before starting off. Said operations consume a large amount of compressed air, which must be produced at a low engine speed.
Owing to the lower transmission ratio, the quantity of compressed air delivered is increased not only at low engine speeds but also at high engine speeds. Owing to their mechanical and thermodynamic design, compressors, which can also be referred to as air pellers or air compressors for example, have a speed limit, above which they cannot be operated reliably. On the one hand, mechanical damage can occur due to extreme loads on the compressor at high engine speeds and, on the other hand, the efficiency of the compressor falls. This applies especially because the nominal speed of the driving engine of approximately 2000 revolutions per minute may be significantly exceeded by the braking speed (approximately 2400 revolutions per minute) in the case of engine braking The problem here is that, when carrying out an engine braking operation, shutting down or relieving the load on the compressor to protect the material is often not desired since compressed air may be required for the service brake to provide additional braking of the vehicle. Indeed, this compressed air consumption should be assumed since the commercial vehicle should be braked already by the engine brake. For efficiency reasons too, shutting down or relieving the load on the compressor during an engine braking operation is furthermore not desired. This is because an engine braking operation represents an “overrun” phase, during which the driving engine is not consuming any fuel, and therefore generating compressed air during an engine braking operation is particularly energy-efficient.
It is the underlying object of the invention to provide a compressor system which resolves this conflict of aims between a maximum air delivery rate at low engine speeds and a limitation of the load on the compressor at high engine speeds with a low outlay on construction.
The present invention consists of a compressor system comprising a planetary gear train having an input shaft and an output shaft and comprising a compressor coupled to the output shaft for producing compressed air, wherein the planetary gear train comprises components that can be moved relative to one another in the form of a ring gear, a planet carrier, and a sun gear, and wherein a first switchable fixing device is provided, by which the ring gear, the planet carrier, or the sun gear can be fixed with respect to an outer support, and wherein a second switchable fixing device is provided, by which two of the components of the planetary gear train that can be moved relative to one another can be fixed with respect to one another. The input shaft and the output shaft can each be connected to one of the planetary gear train components that can be moved relative to one another. At a constant speed of the input shaft, the planetary gear train can provide at least two different speeds at the output shaft, by which the compressor can be driven. This corresponds to two different transmission ratios. In this way, a lower transmission ratio can be provided at a low speed of the input shaft, ensuring that the speed provided at the output shaft for driving the compressor is as high as possible. At a high speed at the input shaft, it is furthermore possible to provide a higher transmission ratio, with the result that the speed provided at the output shaft for driving the compressor is reduced as compared with the lower transmission ratio. To select the transmission ratio of the planetary gear train, the first switchable fixing device and the second switchable fixing device are provided, which can fix parts of the planetary gear train with respect to one another in a manner known per se. The switchable fixing devices can be pneumatically or electrically controllable multiplate clutches, for example. A combination of a synchronizing device and of a simple blocking element is likewise possible as a switchable fixing device. A synchronizer ring in combination with a sliding sleeve and suitable selector teeth is conceivable, for example. The fixing devices can furthermore have one or more actuable electric motors, which allow selective opening and closure of the fixing devices. The fixing devices can be directly or indirectly controllable by an electronic control unit, the compressor system being integrated therein, e.g. by an electronic control unit of the compressed air supply system. In the planetary gear train, the change between different transmission ratios can take place without interruption of the power flow.
According to an advantageous provision, the ring gear can be fixed with respect to an outer support by the first switchable fixing device, and the ring gear can be fixed with respect to the planet carrier by the second switchable fixing device. This special embodiment allows particularly simple implementation of the first switchable fixing device since, in general, the ring gear to be fixed with respect to the outer support encloses the other planetary gear train components that can be moved relative to one another.
Provision can be made for the input shaft to be connected to the planet carrier, and for the output shaft to be connected to a sun gear. In this way, it is possible to modify the transmission ratios that can be provided by the planetary gear train.
Provision can furthermore be made for the compressor system to comprise a control unit which controls the first switchable fixing device and/or the second switchable fixing device. Providing a control unit which controls the first switchable fixing device and/or the second switchable fixing device allows the use of the compressor system described as part of already known compressed air supply systems, it being possible, in particular, to dispense with adaptation of already existing electronic control units of a compressed air treatment system. This allows the use of the compressor system described together with any compressed air treatment systems known to the person skilled in the art.
Provision can usefully be made for the control unit to be designed to fix the ring gear with respect to the planet carrier by actuating the second switchable fixing device when a speed at the input shaft is lower than or equal to a definable threshold speed. Defining a threshold speed is an effective selection criterion for the transmission ratio to be provided by the planetary gear train. The threshold speed can be 1500 revolutions per minute, for example. In this way, the air volume delivered by the compressor at low speeds can be increased.
Provision can furthermore be made for the control unit to be designed to fix the ring gear with respect to the outer support by actuating the first switchable fixing device when a speed at the input shaft is higher than a definable threshold speed. The threshold speed can be 1500 revolutions per minute, for example. In this way, the air volume delivered by the compressor at high engine speeds can be reduced in comparison with the other possible transmission ratio in order to limit the load and the wear on the compressor.
Provision can advantageously be made for the first switchable fixing device and/or the second switchable fixing device to be pneumatically controllable.
As an alternative, it is also possible to make provision for the first switchable fixing device and/or the second switchable fixing device to be electrically controllable.
Provision can furthermore be made for the planetary gear train to be of multistage design. Using a multistage planetary gear train, i.e. a planetary gear train comprising a plurality of planetary gear sets, allows the provision of additional transmission ratios, thus making it possible to adapt the air volume delivered by the compressor in stages to the speed at the input shaft. For example, additional threshold speeds can be provided, delimiting different speed ranges of the driving engine. Each speed range thus defined can be assigned a separate transmission ratio, which can be selected in the presence of an engine speed in the respective speed range. The setting of the selected transmission ratio can be accomplished, for example, by further switchable fixing devices, which can be assigned to the further planetary gear sets. In this way, the air volume delivered by the compressor can be made more independent of the engine speed.
The invention furthermore consists of a method for operating a compressor system comprising a planetary gear train having an input shaft and an output shaft and comprising a compressor coupled to the output shaft for producing compressed air, wherein the planetary gear train comprises components that can be moved relative to one another in the form of a ring gear, a planet carrier, and a sun gear, and wherein the ring gear, the planet carrier, or the sun gear is fixed with respect to an outer support when a speed at the input shaft exceeds or falls below a definable threshold speed, and wherein two of the components that can be moved relative to one another are fixed with respect to one another when none of the components that can be moved relative to one another is fixed with respect to the outer support. In particular, the method can use any compressor systems of the kind described above.
In this way, the advantages and special features of the compressor system described can also be implemented as part of a method.
The method can be developed in a simple manner if the ring gear is fixed with respect to the planet carrier when a speed at the input shaft is lower than or equal to the definable threshold speed, and if the ring gear is fixed with respect to an outer support when the speed at the input shaft is higher than the definable threshold speed.
The advantages and special features in conjunction with the compressor system described in the dependent claims can be implemented in the same way with the method described.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.